The purpose of this study is to describe the relationship between Resting Energy Expenditure (REE), dietary intake and Pulmonary Function (PF) in adolescent females with Cystic Fibrosis. The specific aims are: 1) To describe PF, dietary intake, growth and body composition in adolescent females with CF and unaffected controls. 2) To identify differences in energy balance and body composition in adolescent females with CF relative to unaffected controls in order to describe more thoroughly the nature and degree of malnutrition. 3) To define the relationship between pulmonary function and energy expenditure so that the energy requirements of young women with CF will be better understood. 4) To determine which measure of pulmonary function is most strongly associated with energy expenditure and nutritional status throughou the range of pulmonary function. Disease severity will be assessed by clinical evaluation and Brasfield scoring. Body composition will be detemined by measuring height, weight, skinfolds thickness, and total body electrical conductivity (TOBEC). Determination of lean body mass, fat mass, and bone density will be measured by a three day dietary record. REE will be mesured by indirect calorimetry. Pulmonary function testing will be done by body plethysmography according to the standards of the American Thoracic Society. Thirty adolescent females with CF between the ages of 8 and 20 years will be studied. Thirty sex-matched healthy controls will be selected. To date fifty-four patients, both CF and control subjects, have been enrolled. No complications have been noted.